In general, composite clad metal, such as used for jewelry, contains two or more layers of dissimilar metals bonded together by either atomic or metallurgical bond.
The first precious metal clad of sterling silver on copper known as “Old Sheffield Plate” was commercially realized in the middle of 18th century in England. Since then some precious metal cladding such as gold filled (usually karat gold on brass), karat gold on silver, or other precious metal alloys on base metal have been used for making jewelry as well as for industrial applications, such as electrical contacts—all to achieve two main objectives:                To significantly save on total material cost of precious metals        To combine such unique properties of precious metals such as but not limited to corrosion and tarnish resistance, attractive color, high electrical and thermal conductivity with the desirable mechanical properties of the base metal.        
Several processes are known to produce a strong bond between two or more layers of precious and base metals. All processes involve a combination of pressure and heat:                Cold roll bonding is a simple process where two or more different metal sheets are passed through a rolling mill that creates pressure sufficient enough for establishing a strong atomic bond between layers (FIG. 1).        Hot roll bonding is more effective in achieving a strong bond but also is more complex. In hot roll bonding, the bonding is achieved at least in part due to a high temperature bonding process.        Braze bonding is a process where a layer of solder foil is placed between the layers of metal (FIG. 2), a static weight load is applied to a whole package and the brazing is done in a furnace within a protective atmosphere.        
Silver Metals
In all such clad composite materials, the overall precious metal content historically becomes diluted to a fraction of that of the original precious metal alloy. That is, the precious metal content in one portion is lower than the precious metal in the other portion, and the combined precious metal content overall is less than that required for the jewelry element to be named as consisting of that precious metal. For jewelry applications specifically, certain regulatory requirements, such as but not limited to those in the U.S. Gold and Silver Stamping Act and Federal Trade Commission Guides, require that such clad metals cannot be stamped as solid precious metal alloys when not meeting defined criteria. For example, karat gold clad on sterling silver can be stamped neither as karat gold, nor as sterling silver.
When solder is used for braze bonding, for example, it is a common practice to increase the silver content of the substrate (metal 2) to compensate for solder not being sterling silver. Eventually, during material processing (rolling) the solder layer becomes negligibly thin, and the clad practically remains sterling-on-sterling.
Only the clad metal that consists of two (or more) similar sterling silver metals bonded together satisfies the legal requirement for stamping as solid homogeneous sterling silver. This is especially beneficial when one of the sterling silver metals has an enhanced property such as resistance to tarnish, whereas all other properties such as mechanical properties and color are similar to those of the second metal.
In addition, for an alloy to be markable as “silver”, the alloy must contain at least 92.5% silver.
Only the clad alloy that consists of two (or more) similar same karat gold alloys bonded together satisfies the legal requirement for stamping as a solid homogeneous karat gold. That is, the total percentage of gold determines the karat of the piece. Having a similar amount of gold content in the cladding layer and the substrate layer is especially beneficial in that the gold is, in effect, uniformly distributed. Also, by limiting additional more expensive metals for other purposes, such as palladium for color, to the cladding layer, the total cost is reduced. Further, other than having certain metals in select of the alloys for particular purposes, there is sufficient benefit to having both the cladding and substrate layers having similar mechanical properties. It is desirable therefore, to provide the jewelry industry with the karat gold clad material that combines the properties of both alloys at the same time and that can be stamped as solid homogeneous karat (or silver) alloy in accordance with the requirements of the Stamping Act, FTC Guides, and/or other criteria.
It is desirable therefore, to provide the jewelry industry with a sterling silver clad material that combines the properties of both the substrate and cladding alloys and so that the jewelry element may properly be stamped sterling silver by virtue of it containing a 92.5% silver minimum as if it is a homogeneous sterling silver alloy.
Gold Metals
Recent legislative developments in Europe practically banned nickel containing white karat gold alloys for jewelry applications and left the industry with the practically only option of more expensive palladium containing white karat gold alloys. The use of hardenable palladium containing white golds offers some savings on cost of palladium. The clad of thin layer of palladium containing hardenable white karat gold (similar to those described in U.S. Pat. Nos. 5,919,320 and 7,135,078) on palladium-free hardenable yellow karat gold (similar to that described in U.S. Pat. No. 6,676,776) offers a significant reduction in palladium cost. Such clad also offers all the benefits of hardenable alloys, and satisfy legal requirements for stamping as solid homogeneous karat gold. Moreover, such clad is in line with white gold grading and can be rhodium plated as white gold.
Table 1 below illustrates the reduction of palladium content in the clad. Alloy 1 is from the U.S. Pat. No. 5,919,320, and alloy 2 is from U.S. Pat. No. 6,676,776.
TABLE 1Alloy 1 (top 10% byAlloy 2 (substrate 90% byElementthickness)thickness)CombinedAu58.5058.5058.5Ag18.9013.5014.08Pd12.001.28Co0.200.400.38Zn4.504.02Sn0.150.02In0.150.02Cu10.1023.1021.71
It is also desirable therefore, to provide the jewelry industry with a gold-clad material that combines the properties of both the substrate and cladding alloys and so that the jewelry element may properly be stamped as of a selected gold karat and that the gold distribution is generally consistent between the clad and substrate elements.